Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
The dynamic UL DL sub-frame configuration is being studied in 3GPP and Ministry of Industry and Information Technology (MIIT) of China shows great interest. In the latter, dynamic uplink downlink subframe configuration (referred as dynamic TDD for simplicity later on) is identified as one of the key features to improve the performance in hot spot and indoor scenario.
With dynamic TDD, the neighboring cells can be configured with different uplink downlink subframe configurations. FIG. 1 exemplifies the uplink interference source difference when the two neighboring cells are configured with different UL DL subframe configurations. Cell A and B are configured with TDD subframe configuration 1 and 2 respectively. In Cell A, Subframes 2 and 7 experience the interference from the uplink transmission of the UE in Cell B, while Subframes 3 and 8 experience the interference from the downlink transmission in Cell B.
In downlink, the PDCCH is specified to be transmitted in the first 1 to 3 symbols according to the configured CFI. In uplink, the PUCCH is specified to be transmitted over the side PRBs.
FIGS. 2 and 3 show frame structures of CRS and uplink DM-RS, respectively.                Subframes 3, 8 are configured as downlink subframes for Cell B but uplink subframe for Cell A. For Cell B, in these subframes, the transmitted signal in the control region and the CRS from cell B do not interfere the uplink DM-RS but interfere some data symbols in the uplink of Cell A. Hence, it is difficult to ensure the radio channel quality measurement and estimate accuracy.        Subframes 2 and 7 are configured as uplink subframes in both Cell A and Cell B. In the uplink, the channel quality measurement and estimation accuracy can be ensured since uplink DM-RS symbols experience the similar interference as the data symbols since the UE transmits signals over all the symbols of the allocated PRBs.        
Link adaptation is supposed to adapt the transmit data rate according to the radio condition, available time-frequency resource, buffer status, the predefined parameters and the like, so that the system performance and/or the user experience can be optimized.
A simple example of uplink adaptation is described below. Without impact of describing the invention, it is assumed that the UE has full buffer traffic. Over the scheduled PRBs for the UE, the SINR is measured in every subframe for MCS selection in the coming subframes. In order to maintain a predetermined BLER target, a delta value which is adapted based on the PUSCH CRC check status is used to conquer the measurement error, the variation of the channel and the interference. Then the effective SINR over the allocated PRBs in the current uplink subframe may be expressed as Equation 1:effffectiveSINR=measSINR+Δadapted  Equation 1
Herein, measSINR is the measured SINR over the used uplink PRBs in the current subframe; Δadapted is the said delta value; and effectiveSINR is the expected effective SINR.
The maximum available SINR when certain number of PRBs is allocated may be estimated as Equation 2:
                    effectiveSINR        =                  measSINR          +                      Δ            adapted                    +          PH          +                      lin            ⁢                                                  ⁢            2            ⁢                                                  ⁢                          dB              ⁡                              (                                                      N                                          PRB                      ,                      meas                                                                            N                                          PRB                      ,                      x                                                                      )                                      ⁢                          (                              in                ⁢                                                                  ⁢                dB                            )                                                          Equation        ⁢                                  ⁢        2            
Here, PH is the uplink power headroom which is defined in 36.211; NPRB,meas is the number of used PRBs in the current uplink subframe; and NPRB,x is one of the possible numbers of PRBs that can be allocated in a coming uplink subframe.
As one example, Δadapted can be adapted using a jump algorithm as illustrated in Equation 3. A decrease in a full step size is applied when there is a PUSCH decoding failure and a proportional increase to the BLER target is applied when there is a PUSCH decoding success.
                              Δ          adapted                =                  {                                                                                          Δ                    adapted                                    -                  StepSize                                                                              (                                      PUSCH                    ⁢                                                                                  ⁢                    decoding                    ⁢                                                                                  ⁢                    failure                                    )                                                                                                                          Δ                    adapted                                    +                                      StepSize                    ·                                          BLER                                              1                        -                        BLER                                                                                                                                          (                                      PUSCH                    ⁢                                                                                  ⁢                    decoding                    ⁢                                                                                  ⁢                    success                                    )                                                                                        Equation        ⁢                                  ⁢        3            
In practice, there may be certain optimization considering various factors, for instance the tradeoff between the PRB allocation and the user power efficiency, the inter-cell interference, and the user experience. As one example from the system performance optimization perspective, when there are many users with uplink traffic in the cell, the cell may try to schedule multiple users in one uplink subframe and each of the scheduled users is only allocated with part of the available uplink PRBs. Then, the user can have high effective SINRs and be scheduled with high MCSs compared to a case where only a single user is scheduled in the subframe statistically.
When the dynamic TDD is deployed, the interference characteristics, the radio quality measurement and estimate accuracy between different uplink subframes may be quite different when the neighboring cells are configured with different UL DL subframe configurations. For a certain subframe, when all the neighboring cells configure it as an uplink subframe, the interference in the subframe in each cell may be well estimated since the interference impact from the neighboring cells is included in the channel estimation. When some of the neighboring cells (referred to as Cell Set DL for the subframe) configure the subframe as a downlink subframe but other cells (referred to as Cell Set UL for the subframe) still set the subframe as an uplink subframe, due to the fact that the uplink channel estimation cannot fully capture the interference impact of the CRS and PDCCH from Cell Set DL, Cell Set UL may under-estimate the interference, thereby over-estimating the SINR. This is extremely true when there are only CRS and PDCCH transmitted over the subframe by any one of Cell Set DL.
For certain subframe, in the Cell Set UL, the interfered uplink subframe by the downlink transmission of Cell Set DL can trigger to produce a very conservative delta value when a single common uplink adaptation loop is applied for all the uplink subframes. This may seriously deteriorate the uplink subframes that do not experience the interference from the downlink transmission of any neighboring cells.
FIGS. 4 and 5 show measurements with two indoor pico cells when there are CRS interference and CRS+PDCCH interference from Cell B to Cell A, respectively. The brief configurations for the measurements of FIGS. 4 and 5 may be seen in the following Tables 1 and 2, respectively.
TABLE 1Brief configurations for measurements of FIG. 4RefCase ACell ACell BCell ACell BUL DL subframeDSUUDDSUUDDSUUDDSUDDconfigurationDSUUDDSUUDDSUUDDSUDDtrafficSingle full0 UESingle full0 UEbuffer uplinkbuffer uplinkFTP UEFTP UE
TABLE 2Brief configurations for measurements of FIG. 5RefCase ACell ACell BCell ACell BUL DLDSUUDDSUUDDSUUDDSUDDsubframeDSUUDDSUUDDSUUDDSUDDconfigurationtrafficSingle fullSingle fullSingle fullSingle fullbuffer uplinkbuffer uplinkbuffer uplinkbuffer uplinkFTP UEFTP UEFTP UEFTP UE
It can be seen that the uplink subframes 2, 7 are deteriorated due to the SINR overestimate in subframes 3 and 8 in Cell A, when there is CRS interference from Cell B and there is a single link adaptation loop in uplink.
There are similar issues for the link adaptation in downlink when there is strong UE to UE interference.